linux-kernel - Re: [PATCH -V8 05/16] hugetlb: avoid taking i_mmap_mutex in unmap_single

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Message-ID: <20120609094444.GG1761@cmpxchg.org>
Date:	Sat, 9 Jun 2012 11:44:44 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	linux-mm@...ck.org, kamezawa.hiroyu@...fujitsu.com,
	dhillf@...il.com, rientjes@...gle.com, mhocko@...e.cz,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org
Subject: Re: [PATCH -V8 05/16] hugetlb: avoid taking i_mmap_mutex in
 unmap_single_vma() for hugetlb

On Sat, Jun 09, 2012 at 02:29:50PM +0530, Aneesh Kumar K.V wrote:
> From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
> 
> i_mmap_mutex lock was added in unmap_single_vma by 502717f4e ("hugetlb:
> fix linked list corruption in unmap_hugepage_range()") but we don't use
> page->lru in unmap_hugepage_range any more.  Also the lock was taken
> higher up in the stack in some code path.  That would result in deadlock.
> 
> unmap_mapping_range (i_mmap_mutex)
>  -> unmap_mapping_range_tree
>     -> unmap_mapping_range_vma
>        -> zap_page_range_single
>          -> unmap_single_vma
> 	      -> unmap_hugepage_range (i_mmap_mutex)
> 
> For shared pagetable support for huge pages, since pagetable pages are ref
> counted we don't need any lock during huge_pmd_unshare.  We do take
> i_mmap_mutex in huge_pmd_share while walking the vma_prio_tree in mapping.
> (39dde65c9940c97f ("shared page table for hugetlb page")).
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>

This patch (together with the previous one) seems like a bugfix that's
not really related to the hugetlb controller, unless I miss something.

Could you please submit the fix separately?

Maybe also fold the two patches into one and make it a single bugfix
change that gets rid of the lock by switching away from page->lru.

Thanks
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